1. Field of the Invention
This invention relates to apparatus for the incineration of granular or particulate material, in particular to apparatus for the thermal reclamation of a material such as foundry sand.
Used foundry sand may be subjected to reclamation so that it can be re-used in foundry processes. Such reclamation can take the form of mechanical attrition, whereby the sand is broken down into grain-size particles. However, used foundry sand contains a high proportion of chemical bonding agents, eg phenolic resins, and after a while these agents reach such a level that the properties of the sand deteriorate, even with mechanical reclamation. Consequently, there is a need for a thermal reclamation technique whereby the chemical agents are incinerated, leaving relatively clean sand. Such thermal reclamation is typically conducted in a furnace having a fluidised bed.
In a fluidised bed furnace, material to be treated is fed in and incinerated, the waste gases escaping through a flue stack at the top of the furnace. The reclaimed material is removed either periodically or continuously. The reaction may be substantially self-sustaining. That is to say, in theory at least, once the combustion process has reached a steady state from start-up (typically at around 800.degree. C.), there is no need to supply significant amounts of fuel gas since the combustion is supported by burning of the chemical agents.
2. Description of the Prior Art
Since furnaces of this type are subjected to considerable variations in temperature, yet must be completely air-tight in the fluidised zone, considerable problems are encountered with cracking or even collapse due to thermal expansion. Hitherto, attempts have been made to overcome these problems by constructing the walls of the furnace of specially shaped refractory concrete. Alternatively, the fluidised bed has been enclosed in a stainless steel tank, which may be corrugated, having an external jacket of insulating material, such as ceramic fibre, with minimal thermal expansivity, the remainder of the furnace comprising a steel casing also lined with ceramic fibre.
Furnace designs of this kind suffer from a number of disadvantages. They are, for example, relatively complex and costly to manufacture. In addition, maintenance is difficult and costly to carry out.
There has now been devised an incineration furnace including a fluidised bed which overcomes or substantially mitigates the above-mentioned problems.